A fuel cell is based on an electrochemical cell, wherein two electrodes are in electrical contact with one or more electrolytes that can be solids or liquids. An electrically insulating, ion-permeable membrane may also be situated within the electrolyte. Because the membrane is electrically insulating, electrons formed at the anode are forced to travel through an external circuit back to the cathode to maintain the cathode reaction. Between the anode and the cathode, the electrons can be used to supply power to devices connected to the external circuit or can be fed into an energy storage system such as a battery.
The electrochemical reaction within a fuel cell generates environmentally benign products of electricity, water, and heat from an oxidant source such as oxygen and a fuel source such as, for example, hydrogen. As one specific example, in an alkaline hydrogen fuel cell, oxygen is passed over the cathode to be reduced, and hydrogen is passed over the anode to be oxidized. This oxidation-reduction may occur by several different pathways, depending on the chosen electrolyte and membrane. In an alkaline electrolyte with a hydroxyl-permeable membrane, for example, intermediate hydroxyl ions flow from the cathode, through the membrane, to the anode to be combined with hydrogen. Such an oxidation-reduction may occur through a “four-electron pathway” according to the following reactions:
Cathode side half-reaction (alkaline electrolyte): O2+2H2O+4e−→4OH−
Anode side half-reaction (alkaline electrolyte): 2H2+4OH−→4H2O+4e−
Net reaction: 2H2+O2→2H2O
A less efficient, “two-electron pathway” also is possible, whereby peroxide ions are formed instead of hydroxyl ions, resulting in one part H2O2 as the final product of a net reaction between one part H2 and one part O2. Also known are fuel cells using acidic electrolytes with cation-permeable membranes, such that intermediate ions (protons) flow from the anode, through the electrolyte, to the cathode to be combined with oxygen. An example four-electron pathway in a hydrogen fuel cell with acidic electrolyte involves the following reactions:
Cathode side half-reaction (acidic electrolyte): O2+4e−→2O2−
Anode side half-reaction (acidic electrolyte): 2H2→4H++4e−
Net reaction: 2H2+O2→4H++2O2−→2H2O
The reactions applicable to a hydrogen fuel cell are shown for their relative simplicity. Other fuels and oxidants can be employed in fuel cells, for example, alcohols such as methanol, or complex molecules such as glucose or other sugars. Regardless of the fuel, in any fuel cell employing one of the above four-electron pathways with an oxidant comprising oxygen, the cathode side half-reaction is known as an oxygen-reduction reaction (ORR). Thermodynamics and kinetics of the ORR typically require a cathode catalyst to ensure technically useful output of the fuel cell. Common catalysts for the oxygen reduction at the cathode have included expensive platinum-group metals or alloys.
An efficient catalyst for the ORR at the cathode is essential for practical applications of fuel cell technology. Some of the pioneering work in alkaline fuel cells were for the highly customized and one-time needs of the Apollo Space Program. These alkaline fuel cells included a four-electron ORR catalyzed with platinum-loaded carbon (Pt—C) catalysts. But in general, the use of high-cost noble metal catalysts such as platinum has precluded large-scale commercial applications. In addition to the high costs involved, platinum electrodes in commercial fuel cells may be further limited by issues such as susceptibility of platinum electrodes to time-dependent drift and to deactivation by catalyst poisons such as carbon monoxide (CO). Recent research efforts in reducing or replacing expensive platinum electrodes in fuel cells have led to the development of various Pt-based alloys, of transition metal oxide and organic complexes, of carbon-nanotube-supported metal particles, of enzymatic electrocatalytic systems, and of conducting polymer coated membranes.
However, there remain ongoing needs for efficient ORR cathode catalysts in the development of low-cost, high-performance fuel cells.